1. Field of the Invention
The invention relates to a telemetry device comprising a pill-size ingestible transmitter to obtain deep-body temperature measurements of a human or other subject. It has particular utility in the medical field wherein deep-body temperatures provide an indication of general health.
2. Description of the Prior Art
In the past, deep-body temperature has been measured by hard-wired thermistor ear-probes that must be fitted to the subject's ear in intimate contact with the ear drum. This places the subject in constant discomfort. Thus, U.S. Pat. No. 3,274,994, Sturm, discloses a transmitter inserted into the patient's ear comprising a multivibrator controlled by a thermistor, wherein the multivibrator is activated when a predetermined temperature is reached.
Ingestible transmitters that were first developed were primarily blocking-oscillator telemetry devices using a single transistor as the active element. These devices are simple, but their long-term stability is rather poor and their transmission distances are quite limited. More complicated, stable and accurate transmitter units were later developed for long-term implant. However, these units are large and expensive.
The life of battery-powered telemeters is dependent on the size of the battery, and a large battery cannot be used if the telemeter is to be ingested. A typical battery for ingestible use has a life rating of approximately 16mAH and cannot be remotely controlled. Therefore they constantly draw current from the time the telemeter device is constructed and imbedded in a capsule for ingestion, and must be used immediately if they are to have a practicable period of use.
Passive devices are also known in the art. U.S. Pat. No. 3,229,684, Nagumo et al., discloses a passive telemetering device for physiological measurements which may include a Colpitts oscillator. A thermistor is utilized to vary the time constant variation of a base circuit to affect the oscillating frequency of the transmitter, dependent upon the temperature of the thermistor. Power is supplied to the device by electromagnetic induction from an external source. U.S. Pat. No. 3,407,363, Kaiser et al., also discloses an external power source for a passive implantable FM transmitter, as does U.S. Pat. No. 3,231,834, Watanabe, which relates to a passive telemetering system wherein the transponder response time is proportional to the measured variable.